1. Field of the Invention
This invention relates to a nitride-based semiconductor substrate and, in particular, to a nitride-based semiconductor substrate that satisfies a high transparency and a high conductivity together. Also, this invention relates to a method of making the nitride-based semiconductor substrate.
2. Description of the Related Art
GaN-based compound semiconductors such as gallium nitride (GaN), indium gallium nitride (InGaN) and aluminum gallium nitride (AlGaN) attract attention for a material of blue light emitting diode (LED) or laser diode (LD). Further, since the GaN-based compound semiconductors have a good heat resistance and environment resistance, they have begun to be applied to other electronic devices.
Thus far, the GaN-based compound semiconductors have been grown by using a sapphire single crystal substrate.
However, since the sapphire substrate mismatches in lattice constant with the GaN, a GaN single crystal film cannot be grown directly on the sapphire substrate. Therefore, a method is developed in which a buffer layer (=low-temperature growth buffer layer) of AlN or GaN is grown on the sapphire substrate at a low temperature to buffer a strain in lattice, and then GaN is grown thereon (e.g., JP-A-H04-297023).
By using the low-temperature growth buffer layer, the epitaxial growth of GaN single crystal can be realized. However, the above method still has a problem that the grown GaN has a number of defects since the lattice mismatch between the substrate and the GaN crystal is not perfectly eliminated. It is presumed that the defect may bring some failure to the manufacture of a GaN-based LD and high-brightness LED.
Because of this, a GaN self-standing substrate not to cause the mismatch between the substrate and the GaN crystal is desired to develop. Since it is difficult to grow a large ingot of GaN from a melt by using the pulling method applied to Si or GaAs, various methods such as the HVPE (hydride vapor phase epitaxy), the ultrahigh temperature and pressure method and the flux method and have been tried to make the GaN self-standing substrate.
Especially, the development of the GaN self-standing substrate by HVPE is most advanced. The GaN self-standing substrates by HVPE have gradually begun to be commercially available. Thus, it is much expected to be used for LD and high-brightness LED.
A method for making the GaN self-standing substrate by HVPE is conducted such that a mask with dot-like or stripe windows is formed on a GaAs substrate, growing a GaN buffer layer thereon, epitaxially thereon growing a GaN single crystal layer by HVPE, finally removing the GaAs substrate (e.g., JP-A-2000-12900 and JP-A-2005-213075).
When the GaN self-standing substrate is applied to a light emitting device such as an LED, light emitted from an active layer of LED can be propagated through the GaN self-standing substrate before being radiated externally. Therefore, the GaN self-standing substrate needs to have a high transparency to provide a high efficiency LED.
However, since it is difficult to sufficiently reduce the lattice defect or impurity concentration in nitride-based semiconductors, a nitride-based semiconductor substrate with a sufficient transparency has not been obtained. For example, when a GaN substrate is grown by the conventional HVPE, GaCl3 and NH4Cl, intermediate products in the growth process, with a high moisture absorption property cause the incorporation of oxygen etc. so that the transparency lowers. Further, doping conducted to secure a sufficient conductivity may cause a reduction in the transparency. Thus, it is difficult to satisfy the high transparency and high conductivity together in the nitride-based semiconductor substrate.
The GaN self-standing substrate disclosed in JP-A-2000-12900 and JP-A-2005-213075 fails to have a sufficiently low optical absorption coefficient. Thus, a GaN self-standing substrate with the sufficiently low optical absorption coefficient to provide a high transparency is desired to develop.